1. Field of the Invention
The present invention relates to a light-emitting device, particularly, an organic electroluminescent device, which can emit light by converting electric energy into light.
2. Description of the Related Art
An organic electroluminescent device (hereinafter sometimes referred to as an “organic EL device”) is attracting attention as a promising display device and being aggressively studied and developed because of its capability of high-brightness light emission with a low voltage, but more improvement is demanded on the durability, light emission efficiency, reduction of driving voltage, and the like. As for the light-emitting material, a device containing a tetradentate platinum complex such as phenylpyridine platinum complex and phenoxypyridine platinum complex (see, International Publication No. 05/042550, pamphlet, International Publication No. 05/042444, pamphlet and International Publication No. 04/108857, pamphlet) shows high efficiency and is taken note of. In order to attain higher durability, higher light emission efficiency and lower driving voltage, it is strongly demanded to develop a host material, particularly, an electron-transporting host material, which is combined with such a phosphorescent material. A cyano group is a strongly electron-withdrawing substituent and this substituent is effective for reducing the energy of lowest unoccupied molecular orbital (LUMO) and increasing the electron affinity. As regards a cyano group substituted to an aromatic compound, for example, a perfluorotrisphenylbenzonitrile compound and a production method thereof are disclosed in JP-A-2005-255531 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), but examples specifically demonstrating application to an organic electroluminescent device are not described.
In the specification of JP-A-2006-135160 (laid-open on 2006 May 25, is disclosed a compound wherein a central benzene ring is penta-substituted with a substituted or unsubstituted aryl group or an aromatic heterocyclic groups. However, it does not refer to the effects of a cyano group.
Further, in the JP-A-2003-105332 and the JP-A-2002-329580, is disclosed an aromatic compound having a cyano group as a specific example of the compound. However, no reference is made therein regarding effects produced by an aromatic compound having a cyano group characterized in that a condensed polycyclic aromatic compound or a condensed polycyclic hetero ring is substituted therewith. Also, light emission efficiency is not referred to.